The present invention relates to single cylinder engines such as those widely used for powering lawn mowers, sump pumps, portable generators and the like, and is more particularly related to an engine having a pivoting counterbalance weight that cancels not only the piston acceleration and deceleration forces but also the torsional forces created by the pivoting weight itself in order to reduce torsional vibration forces in the engine.
The basic cause of vibration in a single cylinder engine is piston reciprocation. The piston is started and stopped twice during each rotation of the crankshaft, and reactions to the forces accelerating and decelerating the piston are imposed upon the engine body as vibration in directions parallel to the cylinder axis. In installations such as lawn mowers and the like, the engine produces a vibration that is transmitted through the machine to the operator. While not intolerable for relatively short periods of operation, such vibration is uncomfortable and could produce operator fatigue in cases of continuous operation over an extended period of time. Even in an installation where there is no element of operator fatigue, such as sump pumps or portable generators, engine vibration is undesirable because it causes maintenance problems and tends to reduce the useful life of the machine.
To some extent such vibrations can be decreased by providing the engine with a counterweight fixed on its crankshaft, and located at the side of the crankshaft axis opposite the crankpin by which the connecting rod is connected to the crankshaft. Such a crankshaft counterweight produces a net resultant centrifugal force vector that is diametrically opposite to the crankpin. This centrifugal force vector of the crankshaft counterweight has a component parallel to the cylinder axis that varies as the crankshaft rotates and acts in opposition to the acceleration and deceleration forces on the piston assemblage comprising the piston, its wrist pin and the connecting rod.
If the mass of the crankshaft counterweight is great enough, its force vector parallel to the cylinder axis can cancel the acceleration and deceleration forces on the piston assemblage. Such a force cancelling condition exists when the crankshaft counterweight is of such mass and radius of gyration that its centrifugal force cancels the centrifugal force due to the rotating masses of the crankpin and the crank end of the connecting rod and is large enough so that it also offsets the acceleration and deceleration forces of the piston. Such counter-weighting can be designated as a condition of 100% overbalance.
Unfortunately, centrifugal force due to the crankshaft counterweight also has a component transverse to the cylinder axis by which vibration is produced. As the mass of the crankshaft counterweight is increased, the vibration transverse of the cylinder axis also increases, becoming excessive for practical purposes when the condition of 100% of overbalance is approached. For this reason most single cylinder engines incorporate crankshaft counterweights having a mass that provides a condition of about 50% overbalance so that the centrifugal force due to the counterweight overbalance has a component along the cylinder axis that is equal to about 50% of the acceleration and deceleration forces on the piston assemblage. This represents a compromise between the severe vibration in directions parallel to the cylinder axis that would result with the condition of no overbalance and the severe vibration transverse of the cylinder axis that would result with a condition of 100% overbalance. With the compromise condition of about 50% overbalance there is of course some vibration parallel to the cylinder axis and some vibration transverse to it. Unsatisfactory as it is, the use of crankshaft counterweights that provide a condition of about 50% overbalance is the typical balance system utilized with commercially available engines of this type.
Several balancing systems have been proposed for further reducing vibration producing forces in single cylinder engines. These systems typically utilize a balancing weight pivotally connected to the crankcase and driven in a reciprocal manner by an arrangement that connects the balancing weight to the crankshaft. Examples of such pivoting balancer systems can be found in the following United States patents:
______________________________________ Patent No. Inventor Issue Date ______________________________________ 1,310,090 Ricardo 1919 1,588,720 Gilbert 1926 1,794,715 Knight 1931 1,654,515 Tompkins 1927 3,457,804 Harkness 1969 4,351,202 Summers 1982 4,470,387 Gonska 1984 4,481,918 Morton 1984 ______________________________________
In the pivoting balance systems noted above, the balance weight reciprocally moves through an arc as the piston reciprocates along the cylinder bore axis. With the center of gravity of the balance weight being substantially in line with the cylinder bore axis, this arcuate motion of the balance weight provides the forces required to balance or cancel the piston and rod inertial forces, but it also creates an undesirable torsional vibration of the engine mass, because the force couple required to rotate the balance weight through its angular motion causes torsional acceleration of the engine mass.
U.S. Pat. No. 4,656,981 illustrates another type of balancing system in which a counterbalance weight is not pivoted on the crankcase, but instead is reciprocally driven on a pin projecting from the bottom of the crankcase. The weight is driven by a pair of links journalled on a pair of eccentrics on the crankshaft.